1. Field of the Invention
The present invention relates to a reflective type ferroelectric liquid crystal display and a driving method thereof, more particularly, to a reflective type ferroelectric liquid crystal display in which a transmittance is increased and thus a luminance is increased, and a driving method thereof. The present application is based on Korean Patent Application 2001-62461, filed Oct. 10, 2001, which is incorporated herein by reference.
2. Description of the Related Art
A liquid crystal display is a flat type display, which is widely used for a portable device. Due to the fast development in the scale-up technology, the liquid crystal display is rapidly replacing a conventional CRT (cathode ray tube) display.
There are various kinds of liquid crystal materials applied to the liquid crystal display.
A TN (twisted nematic) liquid crystal, which is generally used as the liquid crystal material, utilizes the interaction between dielectric anisotropy of liquid crystal molecules and electric field, causing several drawbacks like an inefficient display of moving pictures due to a slow reaction time of a few tens of milliseconds (ms), and a narrow visual angle. Also, since cross-talk occurs between the pixels within a certain distance, it is difficult to reduce the pixel size.
Meanwhile, an FLCD (ferroelectric liquid crystal display) utilizes the interaction between spontaneous polarization of ferroelectric liquid crystal and electric field, and provides a rapid response property of 1 ms or lower to display moving pictures without any difficulty. It also provides a wide visual angle. The pixel size, in which cross-talk between pixels does not occur, can be reduced due to the strong interaction between molecules in an FLCD, so that high resolution display is achieved. For the advantages as described above, the field of FLCD has been researched extensively as a next-generation display device.
As one of the ferroelectric liquid crystal materials, which is widely used, there is provided a liquid crystal material of a chiral smectic C-phase (SmC*) having a bistable property and a chevron structure.
In a fabricating process of the FLCD device using the liquid crystal material as described above, the liquid crystal material is injected into a cell between substrates, while being maintained at a desired temperature which is higher than a melting point thereof. Then, when the temperature is dropped, the liquid crystal material of chiral smectic C-phase (SmC*) is transformed to a chiral nematic phase (N*), and then to a smectic A-phase having a layer structure perpendicular to a rubbing direction, and then transformed back to the chiral smectic C-phase. In this process, a long-axis direction of a liquid crystal molecule in a liquid crystal layer is tilted to a desired angle relative to the rubbing direction, reducing the space between smectic layers. As a result, the smectic layer is bent in the liquid crystal layer in order to compensate for a change in volume. The bent layer structure is called the chevron structure, and domains are defined, each having a different long-axis direction according to the bending direction. On a boundary surface between the domains, there is formed a non-uniform orientation having a zigzag defect, a hair-pin defect and a mountain defect.
Due to the orientation property as described as above, a contrast ratio is remarkably lowered. If a DC (direct current) voltage is forcibly exerted in order to prevent the lowering of the contrast ratio, ions within the liquid crystal layer are accumulated on or adsorbed into a surface of an alignment film. Therefore, problems like afterimage effect occur, that is, the previous display pattern is dimly displayed on a current display pattern when a previous display state is changed into the current display state.
Further, a ferroelectric liquid crystal material for providing an AFLC (anti ferroelectric liquid crystal) mode, in which the threshold limit is reduced, is actively being researched. However, since it has a spontaneous polarization value of 100 nC/cm2, the ions are moved due to a depolarization field, and thus the afterimage can be generated. In addition, in the case where an active matrix driving method is applied where the liquid crystal is independently driven in each pixel using a TFT (thin film transistor), the leakage current can be generated by the large spontaneous polarization value. In order to restrict the leakage current, a capacitance has to be increased. However, in this case, since an aperture ratio is reduced, it is difficult to use it as a display device.
To solve the disadvantage of the ferroelectric liquid crystal, the ferroelectric liquid crystal material having a bookshelf structure has been steadily studied where AC (alternating current) driving can be performed and the afterimage is controlled.
There has been provided a ferroelectric liquid crystal having the bookshelf structure, in which the phase transformation is performed without transformation into the smectic A-phase in a crystallization process. That is, when dropping the temperature from an isotropic state of which the temperature is higher than a melting point, the phase is transformed through the chiral nematic phase (N*) and the chiral smectic C-phase (SmC*) in a crystallization process. As one of the liquid crystals in which the phase is transformed from the chiral nematic phase into the chiral smectic C-phase, there is a half-V type liquid crystal having a mono stable property.
In the half-V type liquid crystal, as shown in FIG. 1, when the potential is not applied, an optical axis of the liquid crystal is parallel to the rubbing direction of an alignment film. When the positive potential is applied, the long axis of the liquid crystal is tilted up to a maximum angle of 45°. In FIG. 1, a reference symbol Vsat designates a saturation voltage by which the liquid crystal is maximally tilted.
And when the negative potential is applied, the long axis of the liquid crystal is aligned in a direction which is the same as that of the long axis of the liquid crystal when the potential is not applied. The liquid crystal described above has a relationship between the applied potential and the transmittance, as shown in FIG. 2, i.e., the mono stable property.
Therefore, the liquid crystal has an advantage in that it is possible to perform the AC driving. It is called the half-V type liquid crystal in consideration of the applied potential versus transmittance property.
In the reflective type liquid crystal display in which the half-V type liquid crystal is applied, when the potential is not applied or the negative potential is applied, as shown in FIG. 3, S wave, which is incident to a polarization beam splitter (PBS) 1 and then reflected to a panel 2, maintains a polarized state even after being reflected by a mirror 3 and passing through the panel 2 again. Then, the S wave is reflected by the PBS 1 in the same direction as the light incident direction. In this case, since light is not transmitted in a display direction, perpendicular to the light incident direction, the display state becomes black. However, as shown in FIG. 4, if the positive potential, which is higher than the threshold voltage, is applied, the liquid crystal molecule is gradually tilted corresponding to the applied potential. The S wave, which is incident to the panel 2, is partially transformed into P wave while being reflected by the mirror 3 and passing through the panel 2. A part of the light passes through the PBS 1. The amount of light transmitted through the PBS 1 is increased according to an increase in the applied potential. As shown in FIG. 5, the light amount becomes maximum when the liquid crystal molecule is tilted at an angle of 45°. At this time, since the long axis of the liquid crystal is titled at the angle of 45° with respect to the rubbing direction, the S wave, which is incident through the PBS 1 to the panel 2, is transformed into the P wave while passing through the panel 2 from the mirror 3 in an opposite direction. Therefore, the entire light passes through the PBS 1 and the display state becomes white.
As described above, the half-V type liquid crystal has an advantage of the bookshelf structure. FIG. 6 shows a relationship with the transmittance when performing the AC driving in a cycle corresponding to a data displaying period. In the drawing, a solid line designates the applied voltage and a one-dotted line designates the transmittance.
As shown in FIG. 6, the light is blocked in a negative potential applied region during an AC driving period (T). Therefore, in case the saturation voltage for maximally tilting the liquid crystal is 3V, if a voltage, which is lower than the saturation voltage, is applied, as indicated in a region A during the AC driving period (T), a transmittance of 50% or below on the average is obtained. Further, if the saturation voltage (3V) is applied, as indicated in a region B, a transmittance of 50% on the average is obtained. The potential is not applied in a region C. In this case, the light is blocked. In the conventional half-V type liquid crystal display, as described above, in case the AC driving is performed to maintain stability of the liquid crystal, there is a disadvantage in that only a maximum average transmittance of 50% is obtained during the displaying period.
In order to restrict the light loss, if an asymmetric DC voltage is applied, ions in the liquid crystal accumulates on a surface, thereby generating the afterimage. Further, there is a problem that the liquid crystal is easily degenerated.